Study On Thermal Design For High Power Semiconductor Lasers And Its Impact On The Spectrum

High-power laser diodes(HPLD)are widely used in pumping solid-state and fiber lasers,industrial processing,medical cosmetology,3D sensing,automatic driving and other fields for their compact size,light weight,high efficiency and low cost.Limited by the intrinsic physical properties of the GaAs-based materials and the epitaxial growth process,the electro-optical conversion efficiency of laser diodes is difficult to further improve.As the injection current increases,the heat generation of the chip is severe,the junction temperature continues to rise,and the output power is subject to thermal saturation and rollover.With the increase of heat power,the temperature uniformity of the active region deteriorates,which leads to the broadening of the spectra for high-power laser diodes.The broadened spectrum seriously restricts the application of the laser diodes,especially in pumping solid-state and fiber lasers.Therefore,thermal management is particularly important for high-power semiconductor lasers.This paper focuses on the optimal design of thermal resistance and temperature uniformity for high-power semiconductor lasers,as well as the effect of temperature on spectral broadening.The following research work has been carried out:1.Analytical,three-dimensional(3-D),steady-state,thermal models for high-power single-emitter(SEs)laser diodes and laser diode bars(LDBs)were proposed.Several key assumptions for the models were discussed,and the errors introduced by each assumption and the applicable conditions of the models were clarified.For conduction-cooled SEs and LDBs,the experiments based on wavelength shift method and finite element modeling(FEM)simultaneously verified the accuracy of the analytical 3-D steady-state thermal models as high as 97%.2.Based on the analytical 3-D steady-state thermal models,steady-state thermal characteristics of conduction-cooled SEs and LDBs were studied,and the distributions of temperature field and heat flow for HPLDs were revealed.It was found that for F-Mount SEs and HardCS LDBs,heat flow in the N-side GaAs substrate contributes to 8% and 6% of overall heat dissipation,respectively.Further increasing the thickness and structure of the N-side can effectively reduce the thermal resistance of LDs.It was revealed that test fixtures(representing external cooling conditions)contribute up to 36.8% and 26.0%,respectively,for thermal resistance of F-Mount SEs and HardCS LDBs.3.The concept of differential heat spreading angle and effective heat spreading angle was proposed in this paper.The differential heat spreading angle was used to quantificationally characterize the diffusion process of heat flow in the LD packages.The effect of structural parameters of the conduction-cooled heat sink on the thermal resistance of the LDs was explained by the effective heat spreading angle.The theoretical thermal design curves of conduction-cooled heat sink and submount for LDs subjected to different external cooling conditions were given based on the proposed thermal models.According to the thermal design theory,two kinds of conduction-cooled LDBs with better heat dissipation performance were designed and manufactured.Compared to existed commercial HardCS and MiniCS LDBs,their maximum output power was increased by 12% and 37%,respectively.In addition,under a constant junction temperature rise,the influence of the packages on the temperature difference among the SE active region and between the laser emitters of LDBs was also studied.4.A simplified numerical heat transfer model based on computational fluid dynamics(CFD)for microchannel liquid-cooled HPLDs was established.The convective heat transfer coefficient(HTC)distribution,flow velocity distribution and pressure drop distribution of microchannel heat sink(MCH)under different flow rates were studied.The contribution of each channel section to the overall heat dissipation was revealed.It was found that the HTC at the microchannel inlet is two orders of magnitude larger than the outlet,and the average HTC of the middle channel is three times that of the bottom channel and the upper channel.At a nominal flow rate of 0.3 L/min,the bottom channel and the upper channel contribute equa lly to heat dissipation.The experimental results showed that the maximum error of the CFD numerical model under different flow rates is 10%.Based on the numerical results,an analytical thermal resistance network model for the MCHs of HPLDs was proposed,which characterizes the thermal resistance components contributed from various parts of the MCHs.Compared with the CFD numerical models,the error for calculating the thermal resistance by thermal resistance network is less than 5%.In addition,the MCHs were optimized to reduce the thermal resistance and improve the temperature uniformity,and three MCH design schemes were proposed to reduce the temperature difference between the laser emitters of LDBs.An MCH was designed and manufactured to eliminate lateral thermal diffusion,and the temperature between the laser emitters was nearly 100% consistent for this MCH,which was verified by testing the spatial spectrum of the LDBs.5.The multi-Gaussian function was introduced as the characterization model for the spectra of HPLDs,and the temperature field solved by the analytical 3-D thermal models was coupled into spectral model to study the influence of the injection current,chip structure and package design on the spectral broadening of the SEs and LDBs.The results showed that the spectral FWHM broadening of F-Mount SEs and individual emitters in HardCS LDBs is as high as 75% and 180%,respectively,during normal continuous wave(CW)operation,and the spectral FWHM broadening induced by thermal crosstalk between the laser emitters of HardCS LDBs is as high as 70%.Under a same active region temperature rise,the spectra of SEs broadens as the thickness,width,length of heat sink and thickness of the submount increase.For LDBs,as the thickness of the heat sink increases,the temperature difference between the laser emitters decreases,resulting in a decrease in the spectral width.As the width of the heat sink and the thickness of the submount increase,the temperature difference between the laser emitters of the LDBs increases,which leads to spectral broadening.6.The multi-Gaussian spectral model was used to study the effect of epitaxial process uniformity and package configuration on the spectral broadening of the LDBs.The results showed that the typical epitaxial growth process leads to a broadening of the LDB spectrum of less than 10%.Comparing the package structures of four common commercial high-power LDBs,the packaging residual thermal stress leads to the most serious spectral broadening for HardCS LDBs,and the spectral broadening is only 15%.